The present invention relates to a process for continuously coating metallic bands by physical phase vapor deposition (PVD), and particularly to a process which, by exploiting one or more technological variations of PVD, produces coated metallic bands having high corrosion resistance, adhesion, ductility and weldability features.
The present invention finds its application for the development of very high corrosion resistant coated metallic bands to be used in the automotive, household electrical apparatuses and appliances, and building fields.
Traditionally, metallic bands are coated by a hot dipping process into melt metallic baths or by an electrolytical process.
Dipping processes have the following drawbacks: the kind of coating is essentially limited to zinc or aluminum and their alloys, usually large coating thicknesses, and the formation of intermetallics between the coating and the steel substrate, thus causing brittleness of the same coating.
The electrolytical processes give a better product with respect to the dipping processes, but are limited to the material that can be dissolved into water solution. They have problems from an ecological point of view, connected with the disposal of the electrolytical baths, and finally, they require an electrical power consumption higher than the one required for the dipping process.
Besides the above mentioned processes, vacuum deposition technologies have been developed, such as the vapor phase chemical deposition (PVD) and physical phase vapor deposition (PVD).
By employing batch processes based on these technologies (PVD and PVD), different coatings have been realized, used for specific fields such as the mechanical, aeronautic and energetic fields.
PVD technology can be divided into four categories: evaporation, sputtering, ion plating, ion beam assisted deposition, each one of them comprising many variants.
Evaporation can use different sources, such as: electrical resistance, electronic beam, laser, electronic arc.
Known batch PVD evaporation plants generally employ only one kind of evaporation source, chosen on the basis of the material to be deposited.
Recently an object of the search has been focused on the conversion of batch processes to extend their applicability to the PVD technology to the coating of metallic bands.
It is known that PVD processes are presently under development to continuously coat metallic bands and to be able to confer to the obtained products high corrosion resistance features which have been limited to only zinc coating or aluminum coating.
However, singularly employing these materials in the PVD process, even if high evaporation rates are obtained (zinc reaches 50 xcexcm/s while aluminum reaches 3 xcexcm/s), produces products having corrosion resistance features comparable with those obtained by the conventional hot dipping electro-deposition processes.
Furthermore, it is known that PVD technology of the evaporation category that can be proposed for a continuous process, produces zinc layers on the steel bands having low adhesion due to the presence of an oxide layer on the band surface, which is difficult to remove compatibly with the high speed of the band during the process.
Therefore, there exist numerous needs not yet satisfied for the realization of a continuous PVD process on an industrial scale. Such needs include those of:
obtaining products having corrosion resistance features remarkably better than those that can be obtained by only zinc or aluminum coatings;
obtaining products having adhesion features remarkably better than those that can be obtained by only zinc or aluminum coatings;
having a solution to the incompatibility between the progress speed of the band to be coated in the known industrial lines and the deposition rate of the coating materials.
An object of the present invention is that of providing a process for the continuous production, by physical phase vapor deposition (PVD), of coated metallic bands having a high resistance to corrosion, weldability, ductility, and adhesion of the coating.
Furthermore, the present invention aims to overcome the problems connected with the traditional techniques and the known PVD continuous processes, in order to obtain products with improved corrosion resistance, weldability, ductility, and adhesion features, due to the synergy between the base element of the coating, zinc, and other elements or their compounds present in small quantities.
It is therefore a specific object of the present invention to provide a process for the continuous production of coated metallic bands, characterized in that the coated metallic bands are obtained by physical phase vapor deposition comprising the following steps carried out on a band, eventually coated with zinc or its alloys, in motion and maintained in a vacuum environment:
heating the band to be coated to a temperature included in the range between 250 and 500xc2x0 C.;
eventual activation of the band surface;
heat stabilization of the band, to bring the same to a temperature included in the range between 100 and 250xc2x0 C.;
eventual deposition of the zinc layer on the metallic band;
heat stabilization of the band, to bring the same to a temperature included in the range between 100 and 250xc2x0 C.;
deposition on the zinc, or its alloys, a layer of one or more elements, or of their compounds, able to synergistically interact with the underlying zinc or its alloys layer in order to obtain high corrosion resistance, weldability, ductility and adhesion of the coating properties.
The band preferably advances at a rate between 10 and 200 m/min.
The activation of the not coated metallic band surface can be provided in the process according to the invention. This activation can be carried out by at least one or more of the pre-treatments chosen from the group comprising: electronic beam, mechanical brushing, ion bombing, and reducing atmosphere.
In preferred embodiments of the invention, the deposition on the metallic band of a zinc layer occurs by vaporization through electrical resistance or electronic beam.
The eventual thermal stabilization of the band can occur by heating, through employing an electronic beam or electric resistance, or by cooling with inert gas.
The deposition on the zinc layer or its alloys of one or more elements or their compounds can occur by vaporization by electronic beam.
The vacuum level under which the process is carried out can be included between 1xc3x9710xe2x88x923 and 60xc3x9710xe2x88x923 Pa.
The elements or their compounds to be used for the deposition on the zinc or its alloys layer, can be chosen from among the elements Al, Co, Cr, Cu, Fe, Mg, Mn, Mo, Ni, Pb, Si, Sn, Ti, Y, Zr or their compounds.
The amount of each of these elements or of their compounds into the zinc or its alloys layer is included between 0.2% and 3% in weight.
The deposition of the zinc layer on the metallic band can occur by vaporization creating a conical shaped vapor cloud having a diameter equal to or bigger than the width of the band and the height of the lower cone or equal to 30 cm.
The deposition of the zinc or its alloys layer can occur by vaporization of at least of one of the following elements: Al, Co, Cr, Cu, Fe, Mg, Mn, Mo, Ni, Pb, Si, Sn, Ti, Y, Zr or their compounds, creating a conical shaped vapor cloud with a diameter equal to or bigger than the width of the band and the height of the cone, between 30 and 60 cm.
For the deposition of one of the following elements: Al, Co, Cr, Cu, Fe, Mg, Mn, Mo, Ni, Pb, Si, Sn, Ti, Y, Zr or their compounds on the zinc or its alloys, an electronic beam having a power between 20 and 1500 kW can be used.
The zinc layer deposited on the band by physical phase vapor deposition, or the zinc or its alloys layer deposited by electro-deposition or hot dipping, is at least equal to 0.5 mm.
The present invention also relates to the coated band that can be obtained by the process described above.